tty: clean up legacy leftovers from n_tty line discipline

tty: clean up legacy leftovers from n_tty line discipline

commit 64a69892afadd6fffaeadc65427bb7601161139d upstream.

Back when the line disciplines did their own direct user accesses, they
had to deal with the data copy possibly failing in the middle.

Now that the user copy is done by the tty_io.c code, that failure case
no longer exists.

Remove the left-over error handling code that cannot trigger.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>